Casting metal articles by sand molds is an old process which has developed as an art into practices and procedures too numerous to mention herein. However, for the sake of consistent terminology, at least as to the terminology used herein, every sand mold casting process uses a pattern having an article form shape about which sand is packed so that when the pattern is removed from the mold an article forming cavity is formed in the mold. The mold has two parts, referred to herein as cope and drag, the intersection of which forms the casting parting line. After the sand is packed around the pattern, the pattern is removed and any cores which define cavities in the cast article are inserted into the article forming cavity made by the pattern. The mold is then "reassembled" and the metal poured.
Each pattern has a gating system which includes a sprue, feeders, runners and riser. Minimally, a sprue and a riser are required to be formed in the sand mold when the article forming cavity is such that a thin casting section feeds a thick casting section. The sprue and riser are in fluid communication with the article forming cavity such that liquid metal is poured from the sprue through the article forming cavity into the riser. The riser is termed an open riser when it extends above the mold so that the foundry man can stop pouring the mold when he sees metal in the riser. Risers positioned within the mold and out of sight are termed "blind" risers.
Metal undergoes volumetric contraction when it solidifies. When a casting has thick/thin sections, the thin sections solidify and contract before the thick sections. When a casting has a thick section fed from a thin section it cannot draw metal from the solid thin section to compensate for its volumetric contraction. A riser is thus used to feed metal to the thicker section to avoid shrinkage. Volumetric contraction of castable metals is about five percent. Shrinkage, of course, has nothing to do with hot spots, tears or cracks which occur between thick/thin casting sections because of differential casting cooling rates and which relate, generally, to casting design. Shrinkage, on the other hand, is a foundry art controlled by sprue, runner and riser position. As a matter of conventional foundry practice and irrespective of whether or not a thin casting section is feeding a thick sections, a riser is always provided adjacent a thick section to avoid shrinkage. In some instances where very thin sections in a casting (i.e. fins on a casting) are poured, a runner may directly feed the thin section without a riser attached to the thin section. Solidification is obviously very rapid and metal contraction not a significant problem.
Foundry sand does not possess good insulating characteristics. Densely compacted foundry sand has a K factor (i.e. a heat transfer factor) which varies from 0.6 to 1.2 depending on the density and moisture content. The value of the K factor is such that the foundry sand acts as a chill or heat sink. This means that conventional risers formed in the foundry sand mold must contain a larger mass of metal than what may otherwise be required to insure that the metal in the riser remains liquid until the casting section which the riser feeds has solidified.
The prior art has developed sleeves which are inserted into the mold and which act as risers. The purpose of the sleeve is to keep the metal in the sleeve in a liquid state to feed the thick casting sections. The prior art sleeves are able to do this with less metal than the metal required in a conventional, sand formed riser.
Metal reduction by means of a riser sleeve provides several advantages to the foundry which is not readily apparent at first glance. That is, because the risers are simply cut off from the casting and remelted in the next heat, the initial thought is that there is no practical advantage to be gained by reducing scrap which is simply being recycled. However, risers, especially risers for large size castings, can represent a significant proportion of the weight of the casting. Energy must then be used in the melt furnace to heat metal which is essentially scrap. Further, since a portion of the melt furnace must be used to produce waste, capacity of the furnace is reduced to a level which is less than what is otherwise possible. Also, since the riser mass is larger than what is otherwise possible, thicker riser sections must be removed from the casting which increases the foundry's finishing cost. Also, if the foundry is casting different heats requiring significantly different and tightly controlled alloy compositions, it may not be possible to obtain the desired chemical properties for castings to be poured from a given heat if scrap metal from a prior heat of an incompatible chemistry is used. This could require inventory control for the scrap, further increasing foundry cost.
There are two types of sleeves in commercial use by foundries today. One sleeve is known as an exothermic sleeve. This sleeve is made of foundry sand impregnated with metal particles, such as aluminum and/or iron oxide, which produce an exothermic reaction. The sand, binder and metal particles are formed into a sleeve insulated as a riser in the mold. The underlying theory for such sleeves is that the sleeve itself will supply heat to the riser metal to keep the riser metal liquid. In theory, this would appear an acceptable solution to the problem. However, in practice, it is not. First, before the sleeve can generate an exothermic reaction, the sleeve must be heated to that temperature range whereat the exothermic reaction can occur. Thus, the metal in the riser sleeve must drop in temperature to give up its heat so that the sleeve can be heated. Second, the temperature of the exothermic reaction for the metals which can be economically used in the sleeve is about 2000.degree. F. which is below the liquid point of most castable metals. Thus, the use of such sleeves is limited to foundries other than aluminum or in castings where very large risers must be used. In the latter instance, it is conceivable that the temperature gradient from riser center to riser wall could, in theory, be somewhat affected by an exothermic sleeve to maintain a liquid core. In practice, however, because of the low exothermic temperature, a very large diameter riser sleeve has to be employed. Furthermore, the aluminum oxide and iron oxide can contaminate the foundry sand and sometimes produce agglomerates and/or fines which adversely affect the sand reclamation cycle.
A second type of sleeve which has experienced commercial success is an insulating as opposed to an exothermic sleeve. One such insulating sleeve was pioneered and developed by one of the inventors and was marketed by companies known as Brown Foundry Supplies, Inc. and Brown Insulating Systems, Inc. and is now being marketed today. Because the invention herein can be viewed as an improvement to the Brown liquid riser concept, attached hereto as a part hereof and incorporated by reference herein is Catalog 100 of Brown Foundry Supplies, Inc.; Bulletin 200 of Brown Insulating Systems, Inc.; and two advertisements for Brown Insulating Systems, Inc., which more specifically define the Brown insulated riser.
Generally, the insulated riser is a ceramic sleeve which is inserted as a riser in the sand mold to reduce riser size while maintaining the riser function of preventing shrinkage within the casting. Unlike the exothermic sleeve, the insulating sleeve has a composition which resists transfer of heat by conduction through the sleeve to the foundry sand in the mold which acts as a heat sink. The K factor for the Brown insulated ceramic sleeve is 0.072. By insulating the riser metal, the riser metal stays liquid a longer time than it otherwise would as a mass of metal in direct contact with the foundry sand. Because volumetric contraction upon metal solidification is only about five percent, the metal mass of the riser can be significantly reduced with an insulating riser sleeve.
The ceramic sleeves are used for both blind and open risers. In conjunction with the sleeves there are also provided reducers and caps covering the open end of the sleeve. Also, ceramic sleeves, while typically supplied in cylindrical form, have also been supplied as a truncated cone to achieve maximum metal reservoir with minimum contact area with the article form cavity. Ceramic sleeves and the reducer and cap accessories can be reclaimed and recycled with the foundry sand.
In summary, the insulating ceramic sleeve risers now in use have proven conceptually sound, economically viable and commercially acceptable. However, there are limitations besides the obvious price considerations associated with the sleeves. Ceramic insulating sleeves cannot be used in automatic molding machines which conventionally form risers, runners and sprues from sand. In automatic molding machines, the mold is formed by compressing sand against pattern plates which are carefully removed and in a precise manner, the mold halves are accurately mated, with or without cores, to form the completed mold. The outside diameter of ceramic insulating sleeves cannot be held to the tight tolerances which automatic molding machine applications require when positioning the mold halves and inserting the cores. In addition, the surface of the Brown ceramic insulating sleeves are rough in texture and this further compounds accurate placement of the sleeves in a mold formed by an automatic sand mold machine. Significantly, pressures of 1200 to 1400 psi are typically used in automatic molding machines as the molds are constructed and the cores are set. Ceramic sleeves cannot withstand such pressures and fail.